The interests of this Department of Biochemistry are focused on: (a) pteridine, folate and vitamin B12 metabolism, (b) folate and Methotrexate transport, (c) mitochondrial structure and function, (d) mitochondrial genetics, and (e) heme binding proteins. During the past two decades the various projects in each of these areas have progressed from recording gross observational phenomena, through studies with partially purified extracts, to the current isolation of single proteins or protein complexes, which are completely homogeneous when examined by such techniques as polyacrylamide electrophoresis, isoelectric focusing, ultracentrifugation, gel filtration and absorbance spectrophotometry. It is now possible, therefore, to investigate these purified materials at the molecular level and to relate their mechanisms of action and ligand binding to structure. A prerequisite for the determination of such structure is a knowledge of their amino acid composition. The amino acid content of: (a) rat liver dihydropteridine reductase and phenylalanine hydroxylase; (b) vitamin transport proteins from Lactobacillus casei and L1210 cells; (c) the proteins of mitochondrial ATP synthetases (Complex V), and other oxidative phosphorylation complexes from both beef heart and Neurospora crassa; (d) cytochrome P450, and (e) B12 binding proteins from hog pylorus, gastric juices, and serum are to be determined. Additionally the amino acid residues vital to protein-protein interactions and/or substrate binding of these proteins, hemopexin, and chemically modified dihydrofolate reductases will be identified. Most of these proteins are routinely obtained in homogeneous form in this Department and the resolution of their amino acid compositions will contribute greatly to the understanding of their biological functions in vivo.